Arrangement for demagnetizing a transformer

ABSTRACT

To demagnetize a transformer (TR) in a single-ended forward DC/DC converter with self-driven synchronized rectifiers (V 1 , V 2 ) connected across the secondary winding (N 2 ) of the transformer (TR), a diode (D 1 ) is connected in series with a capacitor (C 1 ) across the primary winding (N 1 ) of the transformer. The diode (D 1 ) transfers magnetization energy stored in the transformer to the capacitor (C 1 ) every time a primary switch (V 3 ) of the converter is turned off. To ensure optimum efficiency of the synchronized rectifiers, a discharging circuit is connected to the capacitor (C 1 ) for discharging the magnetization energy stored therein by drawing a DC current (I) from the capacitor (C 1 ) in response to varying input DC voltage such that complete demagnetization of the transformer (TR) always is attained just before turn-on of the primary switch (V 3 ).

This application is the U.S. National phase of international applicationPCT/SE02/00136 filed 25 Jan. 2002 which designates the U.S.

TECHNICAL FIELD

The invention relates generally to forward DC/DC converters and morespecifically to an arrangement for demagnetizing, i.e. resetting, atransformer in such converters.

BACKGROUND OF THE INVENTION

FIG. 1 shows an embodiment of a known pulse width modulated single-endedforward DC/DC converter with self-driven synchronized rectifiers V1, V2,illustrated as field effect transistors (FETs), connected across asecondary winding N2 of a transformer TR in the converter.

An output filter comprising an inductor L and a capacitor C is connectedacross V2 to provide an output voltage U2 across the capacitor C in amanner known per se.

A primary winding N1 of the transformer TR is connected with one of itsterminals to a (+) terminal 1 of a source of varying input DC voltageU1, and with its other terminal to the drain of a primary switch in theform of a FET V3. The source of V3 is connected to a (−) terminal 2 ofthe voltage source U1. The gate of V3 is pulse width modulated such thatits duty cycle is varied in response to the varying input voltage U1 tokeep the output voltage U2 at a desired value. To accomplish this, theactual value of the output voltage U2 is sensed by a voltage regulator 3and compared to the desired value of the output voltage, that is set inthe voltage regulator 3. In response to differences between the actualvalue and the desired value, the voltage regulator 3 outputs a controlsignal to a control circuit 4. In response to the control signal, thecontrol circuit 4 in its turn outputs a pulse width modulated controlsignal to the gate of V3 to vary the duty cycle of V3 such that theactual value of U2 equals the desired value. During the off period ofthe primary switch V3, the core of the transformer TR has to be reset todischarge the leakage inductance of the transformer TR.

To reset or demagnetize the transformer TR, a so-called snubber circuitis provided in a manner known per se to absorb energy during the offperiod of V3. The snubber circuit comprises a series circuit of a diodeD1 and a capacitor C1 that is connected in parallel with the primarywinding N1 and a resistor R1 which is connected to the terminals of thecapacitor C1. When V3 is turned off, energy which has been accumulatedin the primary winding N1 of the transformer TR is transferred to thecapacitor C1 and dissipated by the resistor R1.

The FETs V1 and V2 are both controlled by the transformer TR in such amanner that V1 is on when V3 is on, while V2 is on when V3 is off. Thus,V2 is on when the transformer TR is being reset. At higher inputvoltages U1, the on periods of V3 will be shorter. Hereby, thetransformer TR will be reset more quickly. This will result in a longerso-called dead time, i.e. the time when there is no voltage across thetransformer TR. As a consequence, V2 will not have any gate drive duringsuch times. Instead, its body diode that generates more losses, willconduct. Hereby, the efficiency of the converter will be lower. Also,the presence of dead time means that the primary switch V3 is exposed tohigher voltage than necessary.

FIG. 2 is a diagram illustrating the voltage UN1 across the primarywinding N1 of the transformer TR versus the time t. The primary switchV3 is turned off at time t1 and is turned on again at time t3. Thetransformer TR is supposed to have been demagnetized at time t2. Thus,the dead time lasts from time t2 to time t3 in FIG. 2. The dead timedepends on the on-time of V3 such that a shorter on-time gives a longerdead time.

To improve the efficiency that is associated with good timing of thesecondary switches, it is possible to control the gate drive of V2 fromthe primary side of the transformer TR. The disadvantages of such asolution are increased complexity and increased costs.

SUMMARY OF THE INVENTION

The object of the invention is to bring about an arrangement fordemagnetizing the transformer in a single-ended forward DC/DC converterwith self-driven synchronized rectifiers to ensure optimal operation ofthe synchronized rectifiers and optimal efficiency of the converter.

This is attained by providing the converter with an arrangement fordemagnetizing/resetting the transformer such that completedemagnetization of the transformer always is attained just beforeturn-on of the primary switch.

Hereby optimal operation of the synchronous rectifiers is achieved aswell as minimum voltage stress of the primary switch.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described more in detail below with reference tothe appended drawing on which FIG. 1, described above, shows anembodiment of a known single-ended forward DC/DC converter withself-driven synchronized rectifiers, FIG. 2, also described above, is adiagram illustrating the voltage across the transformer in the convertershown in FIG. 1, FIG. 3 shows the converter in FIG. 1 provided with anembodiment of a demagnetizing arrangement according to the invention,and FIG. 4 is a diagram illustrating the voltage across the transformerin the converter shown in FIG. 3.

DESCRIPTION OF THE INVENTION

In FIG. 3, components that are identical to those in FIG. 1 have beenprovided with the same reference characters.

To ensure optimum efficiency of the synchronized rectifiers V1, V2 in asingle-ended forward DC/DC converter, in accordance with the invention,the converter is provided with an arrangement fordemagnetizing/resetting the transformer TR such that completedemagnetization of the transformer always is attained just beforeturn-on of the primary switch V3. This is called optimum resetting ofthe transformer TR and is obtained when t_(on)·U1=(1−t_(on))·UC1, wheret,D is the on-time of V3, U1 is the varying input DC voltage, and UC1 isthe voltage across the capacitor C1 in the snubber circuit.

FIG. 3 shows an embodiment of such a demagnetizing arrangementcomprising a discharging circuit for discharging the capacitor C1 with acurrent I depending on the input voltage U1. A higher input voltage U1and a shorter on-time t_(on) of V3 leaves more time for demagnetizationof the transformer primary winding N1. Hence the optimum reset voltageUC1 across C1 must be lower. This is achieved by increasing thedischarge current I from C1.

The embodiment of the discharging circuit in FIG. 3 comprises atransistor T1 that is connected with its emitter to the cathode of thediode D1 via the resistor RI. The collector of the transistor T1 isconnected to the (+) terminal 1 of the source U1. The base of thetransistor T1 is connected to the cathode of the diode D1 via a resistorR2, and to the collector of a transistor T2. The base of the transistorT2 is connected to the (+) terminal 1 of the voltage source U1, and theemitter of the transistor T2 is connected to the (−) terminal 2 of thevoltage source U1 via a resistor R3 and a zener diode D2.

With reference to FIG. 4, the operation of the discharging circuit inFIG. 3 will be described.

The purpose of the discharge circuit according to the invention is toreduce the voltage UC1 across the capacitor C1 when the input voltage U1increases. To accomplish this, in accordance with the invention, C1 isdischarged with a discharge current I≈A·U1+B, where A and B areconstants.

In the embodiment shown in FIG. 3, supposing that the threshold voltageof the zener diode D2 is UZ,I≈(R2/R1)·(U1−UZ)/R3=U1·R2/(R1·R3)−UZ·R2/(R1·R3).

In this case, A=R2/(R1·R3) and B=UZ·R2/(R1·R3).

Thus, when the input voltage U1 increases, the discharge current I willincrease. At the same time, the on-time of V3 is reduced.

The on-time of V3 lasts from t=0 to t=t1 in FIG. 4. At time t1, theprimary switch V3 is turned off. The magnetizing energy in N1, which iskept constant indirectly by the control circuit 4, will be dissipatedmainly in the discharging circuit. However a portion of this energy isstored in the stray capacitances of V3, D1 and N1.

When the voltage UN1 across the primary winding N1 of the transformer TRis 0 at time t1, i.e. when the magnetization of N1 ends, the energy inN1 is independent of U1. At this instant, the demagnetization starts.

The stray capacitances loading N1 will be charged, storing some of themagnetization energy. The remaining magnetization energy is stored in C1and hence dissipated in the discharging circuit according to theinvention. The main part of that energy is lost in the transistor T1.

At time t2 in FIG. 4, the diode D1 is reverse biased and the straycapacitances are discharged. The demagnetization ends when the voltageUN1 across the primary winding N1 of the transformer TR is 0 at time t3.

Consequently, complete demagnetization of the transformer TR is attainedjust before turn-on of the primary switch V3 at time t3.

Thus, by means of the discharging circuit according to the invention,there will be no dead time.

1. An arrangement for demagnetization of a transformer (TR) in asingle-ended forward DC/DC converter, a primary winding (N1) of thetransformer being connected in series with a primary switch (V3) to asource (U1) of varying input DC voltage, a diode (D1) being connected inseries with a capacitor (C1) across the primary winding (N1) fortransferring magnetization energy from the transformer (TR) to thecapacitor (C1) during the off period of the primary switch (V3), and adischarging circuit being connected across the capacitor (C1) in orderto dissipate magnetization energy stored therein, characterized in thatthe discharging circuit comprises means for discharging the capacitor(C1) with such a DC current (I) in response to the varying input DCvoltage that complete demagnetization of the transformer (TR) always isattained just before turn-on of the primary switch (V3).
 2. Thearrangement according to claim 1, characterized in that the dischargingcircuit comprises a first transistor (T1) connected with its emitter tothe cathode of the diode (D1) via a first resistor (R1), with itscollector to one terminal (1) of said source (U1), and with its base tothe cathode of the diode (D1) via a second resistor (R2), and to thecollector of a second transistor (T2) connected with its base to saidone terminal (1) of said source (U1), and with its emitter to the otherterminal (2) of said source (U1) via a third resistor (R3) and azener-diode (D2).